Apparatus for particle size analysis

ABSTRACT

Apparatus for determining size and size distribution of gas suspendable particles in a powder comprising a chamber for enclosing a gaseous medium, such as air, means for suspending particles in the medium, means cooperative with the chamber for sampling at a predetermined rate a portion of the medium and means for sizing and quantitating particles suspended in the sample.

United States Patent [191 Duvall et al.

APPARATUS FOR PARTICLE SIZE ANALYSIS Inventors: Ronald Nash Duvall; Gerald Gold;

Richard La Verne l-lurtle; Blasey Thomas Palermo, all of Elkhart, Ind.

Miles Laboratories, Inc., Elkhart, Ind.

Filed: Jan. 26, 1973 Appl. No.2 326,848

Assignee:

US. Cl 356/36, 356/102, 356/103, 356/244 Int. Cl. G0ln 1/00 Field of Search 356/170, 102, 103, 244, 356/36 References Cited UNITED STATES PATENTS 1/1960 Appel et a1. 356/102 June 25, 1974 3,535,531 10/1970 Neitzel 356/103 3,677,650 7/1972 Klinger 356/103 Primary Examiner-Rona1d L. Wibert Assistant Examiner-Conrad Clark ABSTRACT Apparatus for determining size and size distribution of gas suspendable particles in a powder comprising a chamber for enclosing a gaseous medium, such as air, means for suspending particles in the medium, means cooperative with the chamber for sampling at a predetennined rate a portion of the medium and means for sizing and quantitating particles suspended in the sample.

4 Claims, 5 Drawing Figures Pmmimunzsmn 3819l29 SHEET 2 BF 2 COUNTER PHOTO PULSE MULTIPLIER PREAMPLIFIER HEIGHT COUNTER TUBE DISCRIMINATOR F IG. 5 COUNTER APPARATUS FOR PARTICLE SIZE ANALYSIS BACKGROUND OF THE INVENTION This invention relates to a new apparatus for determining size and size distribution of gas suspendable particles in a powder.

It is desirable to have means for determining the size and size distribution of gas suspendable particles in a powder prior to the use of such powder in a production situation. Small particles, such as those in the subvisual range of about 0.5 micron and on up to those of about 50 microns, which are observable, are known to present numerous hazards in manufacturing operations and to present problems in the preparation of products from powders in which they are present. With some materials these small particles, which are readily gas suspendable, may present serious health hazards to an unprotected person. In other situations, such small particles may cause undesirable cross contamination requiring the destruction of a final product. i

For the formulator, the inability to predetermine the dustiness of a powder has been particularly frustrating. Often, a formula which has been accepted based upon laboratory work has proven to include ingredients that create dust hazards when scaled up for production. Such formula must then be abandoned and the research effort repeated, again without assurance that the formula will be acceptable when scaled up. This heretofore trial and error procedure for developing formulas has resulted in substantial additions to the costs of final products and corresponding increases in the costs to the consuming public.

One method of evaluating a powder hasbeen to visually observe the dust characteristics thereof in the work area in which the powder is being used. Of course, this type of evaluation is subjective and fails to identify the particle sizes and the actual size distribution of particles. Also, particles that are most injurious to health are often subvisual and not identifiable by this method. It is known that areas that visually appear to be substantially free of gas suspendable particles often have many such particles that are capable of causing serioushealth problems.

Another technique of evaluating a powder for gas suspendable particles therein is to drop a portion of powder into a container and measure the amount of material remaining airborne after a period of time. Although this technique is not as subjective as visual observation, it is highly dependent upon manual manipulation and does not provide the important parameter of size distribution of particles within the powder. Accordingly, this method does not provide adequate information for a proper particle size analysis of a powder.

Various air samplers are available for measuring airborne particles, generally under a micron size, in essentially clean environments. Besides being unable to determine the presence of large particles, these air samplers are substantially totally inoperative in the presence of the high number of particles developed by a dusty powder.

SUMMARY OF THE INVENTION This invention is embodied in an apparatus for determining size and size distribution of gas suspendable particles in a powder. The apparatus comprises a chamber for enclosing a gaseous medium, means for suspending particles in the medium, means cooperative with the chamber for obtaining a sample at a predetermined rate of the medium and means for sizing and quantitating particles suspended in the sample.

Accordingly, it is an object of this invention to provide apparatus that quickly and reproducibly determines size and size distribution of gas suspendable particles in a powder.

It is another object of this invention to provide apparatus that determines, in a manner substantially free of health hazards, size and size distribution of particles in a powder prior to the use of such powder.

Other objects of this invention will be readily identifiable by one skilled in the art from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a perspective view showing a preferred embodiment of the invention;

FIG. 2 is a top plan view of a chamber and a sampling means of the apparatus shown in FIG. 1;

FIG. 3 is a cross-section view taken along the line 3-3 of FIG. 2 which includes a cut away portion of a detector therebelow;

FIG. 4 is a top plan view showing the detector of the apparatus of FIG. 1 with the top wall thereof removed;

FIG. 5 is a-schematic block diagram of a counter shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT With the apparatus of this invention it is possible to rapidly and reproducibly determine size and size distribution of gas suspendable particles included in a powder. This apparatus comprises a novel combination of a chamber for enclosing a gaseous medium, means for suspending particles in the medium, means for sampling at a predetermined rate a portion of the medium and means for sizing and quantitating particles suspended in the sample.

Referring now to FIG. 1, a preferred embodiment of the present invention is shown as including a rectangular box-like chamber 13 suitable for enclosing a gaseous medium, a detector 14 having a rectangular boxlike configuration and a three channel counter 15. A tube 16, suitable for providing access for introducing materials, such as powders or gases, into chamber 13, extends through an upper wall portion thereof. As shown in FIGS. 2 and 3, a releasable closure means or trap door 17 is positioned in a normally closed relationship with the inner end of tube 16 within chamber 13. Closure means 17 is frictionally held in this closed position and is releasable therefrom to an open position in which the inner end of tube 16 is substantially unencumbered. Closure means 17 is carried by a transverse rod 19 formed with an external handle portion 18 and can be released from the closed position by rotation of said handle. As illustrated, it is preferred to cover the upper end of tube 16 with a loosely fitting removable cap 20. Means for suspending particles is provided by closure means 17 and the bottom wall of chamber 13.

In the illustrated embodiment of the invention chamber 13 is suitably mounted on top of detector 14, and

means 21 is provided for obtaining from chamber 13 a sample of the medium therein after suspension of particles in such medium. Means 21 includes a first or sampling tube 22 extending through the bottom wall of chamber 13 and the top wall of detector 14, a second tube 23 coaxially surrounding and spaced from first tube 22 over at least a portion thereof and coterminous therewith within detector 14, and suitable gas supply means (not illustrated) for causing a particle free gas to flow through the space between tube 22 and tube 23 into detector 14. A flexible hose 24 is connected to a laterally directed tubular extension 25 formed on tube 23 as well as with the particle free gas means. Tube 22 provides communication between the interiors of chamber 13 and detector 14 such that the medium may be forced or drawn from chamber 13 into detector 14. The upper end of tube 22 terminates within chamber 13 such that, during use of the apparatus of this invention, only particles suspended in the medium in chamber 13 are forcedor drawn therethrough, as opposed to the passage of particles of the powder that bounce upon impact with the bottom wall of the chamber.

The detector 14, which in combination with counter forms the means for sizing and quantitating particles of the apparatus of this invention, is substantially gas tight and light tight. Dectector 14 includes a vent tube 50, the inner end of which faces upwardly and is aligned coaxially with and spaced from the open lower ends of the tubes 22 and 23. In the illustrated embodiment the outer end of vent tube 50 extends laterally through the side wall of detector 14 and is connected by a flexible hose 51 to a suitable vacuum means (not shown).

Inside detector 14, as shown in H6. 4, is a light source 34 which is focused on a collimating mirror 35 which is in turn aligned such that the collimated light therefrom (shown as a dashed line) projects through circularly shaped apertures 37 and 38 in baffles 41 and 42, to form a beam of light of a predetermined size. The beam of light is focused across the coplanar open ends of first and second tubes 22and 23 within detector 14. Preferably this beam is at an angle of 90 to the direction of gas flow from sampling tube 22 to the vent tube 50. The substantially particle free gas flowing through tube 23 is beneficial in that the particle containing gas sample is in effect surrounded by a transparent wall while passing through the light beam, thereby preventing particle contamination of the interior of the detector.

A light activated sensing element 43, in this embodiment consisting of photomultiplier tube, is placed at an angle of about 140 from the line of the light beam. This tube generates a signal or series of pulses proportionnate to the amount of light it receives. For light to reach tube 43 a portion of the light beam must be deflected so that light (shown as a dot-dash line) passes through an aperture 44 in a baffle 45 that shields tube 43 from random light. Such deflection may occur when particles suspended in the medium pass through the light beam, Although an angle of 140 is used for the placement of the photomultiplier tube in this embodiment, other suitable angles may be selected. The portion of the light beam passing completely through the medium is focused on an opening 46 of a light trap compartment 47 the inner surfaces of which are formed of light absorbing material. For additional light absorbing capability, the surfaces of the other interior walls of detector 14 are formed of light absorbing material.

The photomultiplier tube 43 is connected to the three channel counter 15 by a suitable electrical conduit 52 through which the pulses generated by tube 43 are transferred to said counter. A typical electronic circuit suitable for receiving the signal or series of pulses generated by the photomultiplier tube 43 is shown in FIG. 5. In this circuit a preamplifier 53 is provided for receiving and intensifying the electrical signal or pulse. A pulse height discriminator 55, including steering logic and pulse height detectors, transfers the pulse to one of three visual display counters 56, 57, and 58 according to the pulse height.

The following Examples 1 12 describe the calibration and use of the apparatus of this invention. These examples show that accurate evaluations of gas suspendable particles in powders may be made in a short period of time. These examples are only representative of this invention and do not limit the scope or use thereof in any way.

EXAMPLE 1 The detector and counter shown in FIGS. 4 and 5 was calibrated with reference latex spheres having diameters of 0.091, 0.481, 6-14 and 50-100 microns. The reference spheres were packaged as aerosols in which the particles were separated from the propellent. The spheres were sprayed into one neck of a three neck flask as hot gas (air) at a temperature of about F was blown into the second neck. The front wall of chamber 13 was removed and the third neck of the flask was connected by a flexible hose to the first tube 22 of sampling means 21, which tube had an inside diameter of 0.083 inch, so that the particle laden gas flowing therefrom passed through the collimated light beam within detector 14. A vacuum was applied to vent tube 50 through hose 51 and the particle laden air removed thereby. A portion of the light beam was deflected as the particle laden gas passed therethrough. Some of this deflected light struck phototube 43, producing an electrical pulse that was transferred to the three channel counter.

For each size of reference spheres the average pulse height was measured at the input of the pulse height discriminator 55. The discriminator was adjusted so that the first channel thereof measured particle diameters of about 05-10 microns, the second channel measured particle diameters of about ll-50 microns and the third channel measured particle diameters of greater than 50 microns. For this calibration a gas flow of 870 ml. per minute was maintained through tube 22.

For this and the subsequent examples, light source 34 was a tungsten lamp, and photomultiplier tube 43 was a Type IP 28 tube available from RCA.

EXAMPLE 2 11 A 15 gram portion of each of the powders set forth in Table 1 was evaluated with the illustrated apparatus of this invention. In this evaluation the powder to be evaluated was placed in tube 16 with trap door 17 in a closed position so that the powder rested thereon. Cap 20 was then placed over the tube. As the trap door was opened, a timer was started and a vacuum was applied to vent 50. The particles were suspended in the air filling the chamber 13 by the falling of the powder and its striking of the bottom wall of said chamber. Chamber 13 was 6 inches high, 7 inches wide and 4.5 inches deep. Gas together with particles suspended therein were drawn from the chamber 13 through tube 22 of the sampling means into detector 14 where they passed through the light beam before being evacuated through tube 50. The flow rate through tube 22 was maintained at 870 ml. per minute. During the evaluations, room temperature was maintained between 23 and 27C. and the relative humidity of the room did not exceed 30 percent.

A record was made of the count at one minute intervals for each evaluation over a 5 minute period. Nine evaluations were made for each material. It was observed that a substantially steady state was achieved at the fourth minute for the samples. The means of the nine observed counts for each material are set forth in Table 1.

TABLE 1 Gas Suspendable Particles in the 0.5-1O Micron Range Material Time (min) Mean CELUTAB" 25.77 26.33 26.88 26.88

LII-bWN- NU-TAB" 109.77 111.88 112.11 112.11

S.D.Lactose Terra Alba 429.88 439.77 442.22 442.88

Dl-PAC M-PWN Cornstarch U.S.P.

AVICHEL" Magnesium Stearate U.S.P.

Talc U.S.P.

" A highly refined carbohydrate sold under the trademark CELUTAB by Penick and Ford. Limited, Cedar Rapids, Iowa 52406 TABLE 1 -Continued Gas Suspendable Particles in the 0.5-10 Micron Range Material Time (min) b A direct compression binder sold under the trademark NUTAB by Mallinckrodt Chemical Works. St. Louis, Missouri 63160 A direct compacting and tableting sugar sold under the trademark Dl-PAC by American Sugar Company, New York, N.Y. 10005 A microerystalline cellulose sold under the trademark AVlCEL by FMC Corporation. American Viscose Division, Marcus Hook, Pa. 19061 Mean From the observed results it is apparent that the materials having the lowest particle counts should be given preference in product development efforts because in a production operation they would present less dust hazard than those having higher counts.

EXAMPLE 12 Production conditions were simulated by four separate runs with each formula in which 30 Kg. thereof was placed in a 3 cubic foot twin shell dry blender in a room of about 7,600 cubic feet and dropped about 2 feet to the floor of the room. The dust in the room was measured with a Unico Air Sampler, Model No. 600 with a Gelman type A filter for 2 minutes after the material was released. With the Unico Air Sampler the more dusty material yielded a greater weight gain.

The means of the observed results are set forth in It was observed that the dustiness determined with the apparatus of this invention correlated with the readings obtained under simulated production conditions. These results showed that the apparatus of this invention properly predicted what would occur under production conditions.

Accordingly, using the apparatus of this invention a forrnulator may now accurately predict during the development of a product whether or not it will present a dust problem upon production scale-up.

The form of the chamber and suspending means of the apparatus of this invention may be other than the rectangular box-like configuration illustrated. For example, the chamber may be cylindrical. The chamber may also include a vertically rotatable cylindrical suspending means or a vibrationally activated suspending means.

The sizing and quantitating means of this invention may, for example, consist of sieve means. The latter means offering the additional capability of physically observing and analyzing the gas suspended particles after they have been sized and quantitated.

It is understood that the chamber, suspending means, sampling means, and sizing and quantitating means described herein are merely representative. Other suitable apparatus will be evident to one skilled in the art based upon the teachings herein.

What is claimed is:

1. Apparatus for determining, in a powder, size and size distribution of gas suspendable particles therein, the apparatus comprising a chamber for enclosing a gaseous medium,

means for introducing a powder into said chamber consisting of a powder introduction tube extending through a wall portion of said chamber and means within said chamber for releasably closing the inner end of said tube,

means for suspending within said chamber particles of said powder in said gaseous medium,

means for obtaining at a predetermined rate a sample of said medium from within said chamber after suspension therein of said particles, and

means for sizing and quantitating said particles suspended in the sample of said medium.

2. An apparatus according to claim 1 in which said means for obtaining a sample includes a first tube extending from said chamber to said means for sizing and quantitating particles suspended in the sample, a second tube coaxially surrounding and spaced from said first tube over at least a portion thereof and extending coterminously therewith into said means for sizing and quantitating, and means for causing a substantially particle free gas to flow through the space between said first and second tubes for discharge with said sample into said means for sizing and quantitating.

3. Apparatus according to claim 2 in which said means for sizing and quantitating said particles suspended in the sample includes,

means for directing a light beam through said sample,

sensing means displaced from said light beam for sensing light when said light beam is deflected from the normal path thereof, said sensing means generating an electrical signal proportionate to the light received thereby, and

means for receiving the signal generated by said sensing means and displaying the signal in a predetermined manner. 

1. Apparatus for determining, in a powder, size and size distribution of gas suspendable particles therein, the apparatus comprising a chamber for enclosing a gaseous medium, means for introducing a powder into said chamber consisting of a powder introduction tube extending through a wall portion of said chamber and means within said chamber for releasably closing the inner end of said tube, means for suspending within said chamber particles of said powder in said gaseous medium, means for obtaining at a predetermined rate a sample of said medium from within said chamber after suspension therein of said particles, and means for sizing and quantitating said particles suspended in the sample of said medium.
 2. An apparatus according to claim 1 in which said means for obtaining a sample includes a first tube extending from said chamber to said means for sizing and quantitating particles suspended in the sample, a second tube coaxially surrounding and spaced from said first tube over at least a portion thereof and extending coterminously therewith into said means for sizing and quantitating, and means for causing a substantially particle free gas to flow through the space between said first and second tubes for discharge with said sample into said means for sizing and quantitating.
 3. Apparatus according to claim 2 in which said means for sizing and quantitating said particles suspended in the sample includes, means for directing a light beam through said sample, sensing means displaced from said light beam for sensing light when said light beam is deflected from the normal path thereof, said sensing means generating an electrical signal proportionate to the light received thereby, and means for receiving the signal generated by said sensing means and displaying the signal in a predetermined manner.
 4. Apparatus according to claim 1 in which said means for sizing and quantitating said particles suspended in the sample includes, means for directing a light beam through said sample, sensing means displaced from said light beam for sensing light when said light beam is deflected from the normal path thereof, said sensing means generating an electrical signal proportionate to the light received thereby, and means for receiving the signal generated by said sensing means and displaying the signal in a predetermined manner. 